Many bacterial diseases are known which severely affect growth and productivity of many species of cultivated plants. The economic damage caused by such diseases is considerable and effort is put into the control of bacterial diseases For example, in 1991 the effort to control fireblight (Erwinia amylovora) in pome fruit (apples and pears) resulted in pesticide applications to an estimated 677,000 product acres. None of the agents currently available to combat bacterial disease in plants are completely satisfactory. Antibiotics and bactericides, such as streptomycin and copper containing pesticides, respectively, normally give limited control. Copper compounds were the first chemicals used commercially for fireblight control. A variety of compounds and formulations are available, including a mixture of copper hydroxide and sulfur (Kocide 101), copper oxychloride sulfate (COCS), and various other inorganic and organic copper compounds. The classic bordeaux mixture has been used extensively. As a group, copper compounds are less effective in controlling fireblight and are more phytotoxic than antibiotics. Most copper compounds cause leaf chlorosis or necrosis and fruit russetting when applied to pear or apple orchards; severity depends on the compound used, timing with respect to stage of growth, formulation and concentration used, and the variety of trees treated.
Antibiotics are antimicrobial compounds that are produced by other microbes. They are produced in quantity either by growth of the organism that synthesizes them or through chemical synthesis. Some have questioned the advisability of using antibiotics for plant protection, because the same materials are sometimes used in human and animal medicine. Widespread use of antibiotics may lead to the development of bacteria that are resistant to the antibiotics; conceivably that resistance may be transferred to bacteria of medical importance.
In order to have a better control of bacterial plant diseases, plant growth regulators, particularly plant growth retardants (i.e., compounds which reduce longitudinal growth), have been investigated on several instances. The degree of success has been relatively limited and very inconsistent. Wilt diseases such as fungal disease on tomato or cotton have been reduced when plants were treated with chlormequat chloride (Sinha and Wood, 1964, Nature 202:824; Erwin et al. 1979, Californian Agriculture 33:8). In other studies chlormequat chloride applications were superior to bactericides for control of Xanthomonas on peppers (Crossan and Fieldhouse, 1964, Plant Disease Reports, 48:549). Likewise, the severity of disease caused by Pseudomonas, P. syringas pv. hibisci, by treatment with chloromequat chloride (Chase et al., 1987, Plant Disease 71:186).
Contrasting with above mentioned reports, others have found that the severity of bacterial diseases was increased under the influence of the growth retardant chlormequat chloride: Xanthomonas malvacearum in cotton (Hiremath et al., 1973, Myosore Journal of Agricultural Sciences 7:565) and Xanthomonas campestris pv. vignicola in cowpea (Panduranga and Hiremath, 1986, Indian Phytopathology 39:512). Likewise, Deckers and Faust (Acta Horticulturae 322:293, 1992) and Deckers and Daemen (Acta Horticulturae 338:205; 1993) presented evidence for an increased susceptibility of pear trees toward infection with fireblight (caused by Erwinia amylovora) after being treated with the growth retardants chlormequat chloride, flurprimidol or triazole type compounds.
Others have found that an acylcyclo-hexanedione type plant growth regulator does not suppress fungal diseases in turfgrass. The use of CGA 163935 (trinexepac-ethyl) did not reduce the severity of necrotic ring spot (Leptosphaeria korrae) on bluegrass; nor the severity of spring leaf spot (Drechslera poae) on bluegrass; nor the severity of Rhizoctonia blight (Rhizoctonia solani) on tall fescue (Sanders and Soika, 1992, Fungicide and Nematicide Tests 47:301, 47:300 and 47:309). In the latter two reports, the use of CGA 163935 in combination with the fungicide Banners.RTM. was less effective than the fungicide used alone.
Prohexadione belongs to a new family of plant growth regulators (acylcyclohexanedione type plant growth regulators). These growth regulators block the biosynthesis of gibberellin (GA). Gibberellin is mainly responsible for controlling cell elongation. When gibberellin biosynthesis is blocked, plant cells will divide normally but the cells will be shorter. This results in shorter plants (reduced stature). Inhibitors of gibberellin biosynthesis are used in many crops to reduce stature, prevent lodging and the like.
No information to date has been available on the actual effects of this new group of growth retardants, the acylcyclohexanediones, on bacterial plant diseases.
U.S. Pat. No. 4,560,403 describes Prohexadione (3-hydroxy4-propionyl-5-oxo-3-cyclohexene carboxylic acid) and a number of other compounds of a class of cyclohexene plant growth regulators. While the reference does suggest as a general principle that as plant growth regulators all the described compounds could be useful to promote resistance "to phytotoxicity caused by . . . bacterial diseases." It appears that due to the lack of consistent results, both positive and negative, with plant growth regulators; the large number of compounds disclosed in the prior art; the lack of any test data in the reference which would show antibacterial activity; a lack of susceptible organisms and plants disclosed; a lack of effective dosages; that the reference is merely an invitation to experiment to see if resistance can be promoted and not predictive or otherwise of a compound for inducing resistance to bacterial disease for plants.